chore: Add @export annotations to polyfill install methods

[vodlogic]

Description

This change allows polyfills to be installed individually e.g. shaka.polyfill.MediaCapabilities.install() instead of shaka.polyfill.installAll()

Related to #2625

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

Checklist:

• I have signed the Google CLA https://cla.developers.google.com
• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have verified my change on multiple browsers on different platforms
• I have run ./build/all.py and the build passes
• I have run ./build/test.py and all tests pass

[google-cla]

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[vodlogic]

@googlebot I signed it!

[shaka-bot]

All tests passed!

[joeyparrish]

Thanks!